I havnt done the beaches there, but the aquarium at Atlantis is awesome!! The tour goes fast but afterwards you can go back through at your own speed as many times as you want at your own pace. Just leave your wrist band on. Kids really seemd to like this.
I think there is a public beach just a couple blocks from there.
Also there is a horse and buggy ride right at the port, $10 per person and it lasts about 20 to 25 minutes. It is fun because it is something you dont normally do. I saw lots of families doing this, dont think there is any age limit.
It is a little noisy due to traffic, but it is fun.
The oldest kid may like the Pirate museum but probably not the little one. it is $12 per person, takes about an hour and it a short walk from the ship. A large purple building, 2 blocks behind the straw market. The gift shop has several neat pirate things to buy too and reasonable priced.
one tip, what ever you do, watch where the restroom are. Not many around and NO water fountains, so take water from the ship and get some travel size T paper and seat covers, you will need them, plus hand sanitizer.
There are restrooms in the port building and in the pirate museum about 1/2 way through, of course Atlantis has nice restrooms. But most areas they are not so nice and hard to find.
mom x4,grandma x4